Do any of the following:
You can change the default theme to a theme that is included with PowerPoint or a theme that you create yourself.
If you haven't already, create and apply the theme that you want to set as the default.
For more information about creating a theme, see Create and apply your own theme.
On the Themes tab, under Themes, click the arrow next to Save Theme, and then click Set Current Theme as Default.
The next time that you open a new presentation, it will open with the theme that you set as the default.
On the Themes tab, under Themes, click the arrow next to Save Theme, and then click Reset Default to White Theme.